This invention relates to coal bed methane gas. More particularly, but not by way of limitation, this invention relates to a method of in situ producing methane gas from underground coal seams.
While coal has long been recognized as a major source of energy, it also contains vast quantities of methane gas. The gas is believed to have been produced during the transformation of vegetation to coal during geologic time and retained within the coal seams by virtue of their low permeability and diffusivity. This methane source is not new, since coal mine operators have been acutely aware of its presence. In fact, during mining operations, the release of the methane into the atmosphere has become an important safety factor and environmental concern.
The difficulty of harnessing the methane gas from these coal seams has made it difficult to assess the commercial potential of methane gas. Nevertheless, significant reserves of methane gas are known to exist. Methane gas occurs in an adsorbed form in the coal seam.
Studies have indicated that a ton of anthracite today occupying a volume of less than 30 cubic feet have generated in the order of 10,000 standard cubic feet of methane during its lifetime. It is estimated that the surface area of one pound of coal is from 100,000 sq. ft. to in excess of 1,000,000 sq. ft. Coal containing 27% volatile matter can absorb (at 77 deg) up to 640 scf/ton with a monomolecular layer of methane molecules, at 4 angstroms in diameter. Gas is stored primarily by sorption into the coal, also gas is stored in the pore or cleat space. The United States Bureau of Mines has collected data that have indicated a gas-in-place magnitude between 300 and 400 Tcf from a combination of minable and unminable coal seams.
(Coal deposits are naturally fractured gas reservoirs. In fact, the methane found in the coal seam reservoir is the same as methane found in the petroleum industry's sandstone and carbonate reservoirs; consequently, a coal bed methane well reservoir is similar to a dry natural gas from a conventional gas reservoir. Coal gas is almost 100% comprised of methane, with just trace amounts of other gases.
Wells completed in coal deposits go through three distinct production stages. The first stage includes the production of trace amounts of gas and in situ water. During this initial stage, the production rates of both products are essentially constant. Generally, the water production rate is the highest rate that the well will ever see. Periodically, it is necessary to pump by mechanical means the water out of the wellbore as a way to produce in-situ water and gas. The methane production rate is initially characterized by a low rate; however, the methane does increase at a relatively constant rate. The first stage may last only a short time in comparison to the overall life of the well; hence, many first stage productions last from two to six months. The wells must be de-watered so as to reduce the hydrostatic pressure on the coal face. This reduced hydrostatic pressure will allow the methane to diffuse from the coal face.
The second stage is characterized by rapid water production decline, and simultaneously, increased methane production. The water withdrawal continues for a period of time, for example for months, while adsorbed methane is desorbing from the micropores of the coal face and begins to flow into the fracture system that is ultimately connected to the wellbore. The desorbed methane production will begin increasing during this time.
The third stage is defined by maximum rate of the gas production and a markedly reduced water rate. Nevertheless, water must still be pumped through out the life of the well. This has been referred to by those of ordinary skill in the art as the "reverse decline curve". The negative decline continues ascending for an extended period of time, for example 30 years, depending upon well spacing of offset wells and the height and size and gas content of the coal seam. The third stage spans most of the productive economic life of the coal bed methane well.
During this third stage, the coal bed methane well behaves as a dry gas reservoir with the only difference being that the gas is stored in the coal bed by sorption in the coal matrix. The dewatered coal bed methane gas is produced at a slightly declining base line that can last for years. The well must be periodically dewatered so as the methane gas can continue to flow and the gas be placed in a pipeline and sold.
Prior art methods of production include providing a bore hole, which is generally vertical, to a depth that intersects a plurality of coal seams. The bore hole depths normally range from 1250' to 7500'. Coal deposits are naturally fractured gas reservoirs. Initially, the natural fractures, or cleats, of the coal are typically water-saturated and the coal matrix adsorbs most of the gas. Most of the storage of gas in coal is by adsorption into the coal structure, while the coal permeability is cleat fractured.
Despite all these advances, the optimum production rates may take some time to achieve. Further, as is applicable to natural gas subterranean reservoirs, operators continue trying to obtain maximum production rates that will maximize the sales volume, while at the same time not harm the ultimate reserve potential of the coal seam. Therefore, there is a need for a method and apparatus to increase and stimulate production of the methane gas, as well increasing ultimate recovery.